In the Head Up Display function, the virtual image is placed in the field of view of the driver taking into account the ellipsis of his eyes and the real scene. It is, however, necessary to be able to adjust the position of the virtual image about its nominal position the better to adapt it to the height of the driver's seat, to his height and to the architecture of the vehicle.
In head up display devices based on the use of a series of minors, the position of the virtual image is adjusted by rotation of one (or more) mirrors about an axis. This solution is not applicable in the case of a diffractive head up display device having no minors and using diffractive components, in particular when the diffractive combiner has been made from a process for recording the diffractive elements which by design fixes the downward angle of view of the virtual image produced by the display device for a determined angle of illumination of the combiner.